Digital audio switching power amplifiers, also called class D amplifiers, are used in various devices, such as in audio systems to drive one or more speakers. Digital audio switching power amplifiers operate in the digital domain using digitally generated pulse width modulated (PWM) signals rather than the traditional analog signal generated using digital-to-analog conversion. More specifically, a digital input audio signal is converted into a digital PWM signal. Typically, the PWM signal contains pulses having a constant frequency and duty cycles proportional to the amplitude of the instant input signal. In other words, the width of the PWM pulse varies with the quantized amplitude of the input signal. The PWM signal switches power transistors between their ‘on’ and ‘off’ states to produce an amplified PWM signal. The amplified PWM signal is typically filtered using a passive LC (inductor, capacitor) low pass filter to generate an equivalent analog output for the speakers connected to the amplifier.
The digital switching nature of switching power amplifiers can make it difficult to power them up or down without an audible ‘pop’ or ‘click’. This occurs when the system is powered up/powered down due to, for example, DC transients in charging/discharging of a coupling capacitor disposed between the amplifier and the speakers to the operational DC bias level. To avoid audible artifacts during power up/down, all DC transients that can reach the speaker are typically slowed down to sub-audible transition rates. However, the operational DC bias level of the amplifier is not exactly predictable. In particular, the DC bias level of a switching amplifier that incorporates a feedback circuit for correcting power stage non-idealities depends upon component matching of both integrated and/or non-integrated circuit components, making it very difficult to accurately predict the operational DC bias. Therefore, a method is needed to gracefully power up and power down a closed loop digital switching amplifier so as to avoid audible artifacts.